Surrounding the north polar region at greater distance are the so-called Olympia Undae, a vast dune field covering an area of 470 000 km² from 78° to 83°N latitude (larger than Sweden, about the size of the whole Balkan peninsula). Already in 2005, shortly after the arrival at Mars of the Mars Express orbiter, the OMEGA instrument detected high gypsum concentrations on the dunes of Olympia Undae. These dunes are intriguing due to their unique gypsum composition, proximity to the polar ice deposits, and seasonal variations.